The present invention relates to a reduction ratio control for a continuously variable transmission for a motor vehicle, and more particularly to a reduction ratio control after the motor vehicle has started deceleration.
Laying-open Japanese Patent Application 58-170958 discloses a reduction ratio control for a continuously variable transmission. In this known reduction ratio control, the discharge of hydraulic fluid from the driver pulley starts upon receiving a brake operation signal generated after depression of a brake pedal, causing the transmission to shift down toward the largest reduction ratio. Laying-open Japanese Patent Application 58-170959 discloses another reduction ratio control wherein the transmission starts shifting down toward the largest reduction ratio immediately after the throttle valve has been fully closed.
Since, according to these known reduction ratio controls, the transmission starts shifting down upon receiving a change in operating condition which change is empirically considered to take place before the driver demands engine brake running, there is an advantage that, even with the continuously variable transmission that is poor in ratio change response, the engine brake running is effected quickly as desired.
However, the speed at which the continuously variable transmission shifts down is not quick enough so that the transmission cannot establish the largest reduction ratio before the vehicle comes to a halt if the vehicle is subject to quick brake operation to such an extent that the road wheels are locked, causing the transmission output shaft to stop its rotation. If the road wheels are locked during brake operation, the vehicle speed indicated by the vehicle speed indicative signal becomes zero. This urges the transmission to effect shifting down further toward the largest reduction ratio, but since, when the transmission output shaft and in turn the driven pulley is not in rotation, the ratio change response is very poor. If the accelerator pedal is depressed immediately after this shifting down operation has started by discharging hydraulic fluid from the driver pulley, the transmission V-belt slips, failing to transmit torque, causing the V-belt to wear at a quick rate. This is because the friction between the V-belt and the driver pulley is lost when hydraulic fluid is being discharged from the driver pulley for effective shifting down operation.
In addition to the above problem, there is another problem that since the shifting down operation takes place immediately after a change in operating condition in anticipation of need for engine brake running, the transmission may shift down even if the driver steps on the brake pedal without any intention of engine brake running. In this case, the vehicle's performance fails to correspond to the driver's feeling.
An object of the present invention is to provide a reduction ratio control for a continuously variable transmission which ensures that the transmission establishes the largest reduction ratio before the vehicle comes to a halt, preventing the V-belt from slipping upon application of torque to the transmission even after the vehicle has been subject to quick brake operation.